1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to a pneumatic tire having improved rolling resistance and wear resistance by constituting a tread portion with a particularly rubber composition.
2. Related Art Statement
It is well-known that tires for automobiles causes self-heat generation thereinside during the running. Such an internal heat generation increases the rolling resistance of the tire. Recently, it is largely demanded to decrease the rolling resistance of the tire from a viewpoint of energy-saving and resources-saving, and consequently the need on rubber compositions having a low heat generating property is more increased.
In general, in order to provide a rubber composition for tire tread or the like having a high fracture resistance, it is desired to use a carbon black having a high specific surface area. The specific surface area is usually determined by calculating from an adsorption amount of nitrogen gas molecule (hereinafter referred to as N.sub.2 SA).
However, when a carbon black, having such a high specific surface area that N.sub.2 SA exceeds about 120 m.sup.2 /g, is used as a filler for reinforcing a elastomer such as rubber or the like, an interrelation between specific surface area and reinforcing performance is not established, and the reinforcing performance remains lower than the expected value. Furthermore, when N.sub.2 SA exceeds 140 m.sup.2 /g, the reinforcing performance is rapidly lowered in some cases.
This is considered to be due to the fact that as the specific surface area becomes high, the dispersibility of carbon black into the elastomer phase degrades and the reinforcing performance inherent to the filled carbon black can not sufficiently be developed and hence the effective bonding state to rubber can not be formed.
In addition, when the rubber composition is used in a site repeatedly subjected to deformation such as tire tread or the like, it is simultaneously required to have a hysteresis loss accompanied with the repeated deformation or a proper dynamic viscoelasticity together with the aforementioned wear resistance.
The inventors have already found that the dispersibility into rubber is improved without the degradation of the reinforcing performance by specifying a ratio of iodine adsorption value to N.sub.2 SA and an existing ratio of a relatively large size portion of aggregate size in carbon blacks having a very large specific surface area (N.sub.2 SA: 200-280 m.sup.2 /g), which has previously been filed (Japanese Patent laid open No. 61-34072).
On the other hand, in order to provide a rubber composition having a low heat generation, it has hitherto been attempted to reduce an amount of a softening agent such as oil or the like, or to change or reduce a reinforcing agent such as carbon black or the like.
However, the aforementioned carbon black proposed by the inventors has the drawback that the use range is extremely restricted because the specific surface area largely exceed the value of the conventional wear-resistant carbon black. Furthermore, the dispersibility is improved, but the improvement of dynamic viscoelasticity is still unsatisfactory.
On the other hand, the reduction of the amount of the softening agent considerably increases the viscosity of the rubber composition, which brings about the degradation of the processability such as kneading or the like. Further, the reduction of the amount of the reinforcing agent or the change to a reinforcing agent having a small reinforcement undesirably decreases a modulus of elasticity, fatigue resistance, particularly wear resistance for rubber.
Thus, there is no technique simultaneously and sufficiently establishing the properties such as low heat generation, good wear resistance and processability up to the present.